Make sure F6 focus cycle falls on focusable view

Fixes regression after refactoring of DownloadItemView where the new
view implementation is not focusable (but a subset of its children are).

DownloadShelfView returns a DownloadItemView as its first focusable
item (totally reasonable), but since the item itself is not focusable we
need to forward focus to the open button inside it.

I still think it's reasonable to focus an unfocusable item if its
children are focusable, so ideally a fix would change ::SetFocusedView
to focus the first focusable child (if any) or otherwise clear focus.
There's a TODO present inside AccessiblePaneView to address this.

Bug: chromium:1000998
Change-Id: I071d1c59401b35f251cd429ae3b5892462f91adf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1798086Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695678}

ui/views/accessible_pane_view.cc

@@ -77,6 +77,15 @@ bool AccessiblePaneView::SetPaneFocus(views::View* initial_focus) {
 
   focus_manager_->SetFocusedView(initial_focus);
 
+  // TODO(pbos): Move this behavior into FocusManager. Focusing an unfocusable
+  // view should do something smart (move focus to its children or clear focus).
+  // DownloadItemView is an example (isn't focusable, has focusable children).
+  // See https://crbug.com/1000998.
+  // The initially-focused view may not be focusable (but one of its children
+  // might). We may need to advance focus here to make sure focus is on
+  // something focusable.
+  focus_manager_->AdvanceFocusIfNecessary();
+
   // If we already have pane focus, we're done.
   if (pane_has_focus_)
     return true;